Developmental programs must impose cell-type-specific controls on cell cycle progression for normal embryonic development, tissue renewal and repair, and to prevent cancer. A striking case is meiosis, the signature event of germ cell development. Meiosis requires a specialized cell cycle (meiotic prophase) in which progression through G2 is slowed or arrested to allow time for the chromosomal events and gene expression programs that set the stage for formation of haploid gametes. We are using the powerful genetic tools and accessible cell biology of Drosophila spermatogenesis to investigate the regulatory mechanisms that specify and control progression through meiotic prophase in males, which is regulated differently than in females. We found that developmentally programmed translational control regulates expression of core cell cycle machinery proteins during male meiotic prophase by two independent pathways: translational repression by a male germ- cell-specific hnRNP A homolog Rbp4 in young spermatocytes delays expression of Cyclin B protein, and translational repression by as yet unknown mechanisms and activation by the RNA binding protein Boule (homolog of human BOULE and DAZ) regulate translation of the cell cycle phosphatase cdc25/twine. To discover the underlying mechanisms through which the germ cell program regulates meiotic cell cycle progression, we will investigate how Rbp4 and its partner Fest repress translation of cycB, testing models suggested by interacting proteins we have identified by mass spectrometry. To understand how the translational repression of cycB by Rbp4 is reversed in mature spermatocytes, we will investigate how interactions among the repression complex proteins and with the cycB 3'UTR change as spermatocytes progress from early to late meiotic prophase, taking advantage of a novel technique to trigger spermatogonia to initiate the spermatocyte program and progress through meiotic prophase in synchrony in vivo, and probe how this activation depends on the hnRNP Q homolog Syncrip (Syp). On the other arm of the pathway, we will investigate how the RNA binding protein Boule, conserved from flies to man, relieves translational repression of cdc25/twine to activate the G2/M transition for meiosis I and probe the role of Boule-interacting proteins in activation and action of Boule. We will map the RNA regulatory regions that specify translational repression of the cell cycle phosphatase cdc25/twine in young spermatocytes and test whether the hnRNP L homolog Smooth blocks premature expression of cdc25/twine indirectly by sequestering Boule to the nucleus, or acts directly on the mRNA to repress translation in immature spermatocytes, and whether other candidates, such as the Pumilio family member CG11123, act as translational repressors. Our findings will illuminate the mode of action and regulation of conserved RNA-binding proteins and hnRNP homologs in cell cycle control and suggest mechanisms to test for similar function during mammalian spermatogenesis.